Warren Harmon Lewis 465 
times, in the somewhat irregular transplanted eyes, the optic nerve 
may divide as it passes through the retina, one branch passing out into 
the mesenchyme and the other entering the pigment layer. 
Fig. 14, from experiment DF,, shows a very remarkable condition, in 
which the transplanted eye touches the medulla in the region of 
the choroidal fissure. The optic nerve passes into the medulla. Fig. 
15 shows an even more remarkable example of the course of an optic 
nerve from the eye to the medulla. The position of the choroidal fissure 
is on the ventral side of the transplanted eye and some distance from 
where the eye nearly touches the medulla. The nerve leaves the eye 
at the fissure, passes at first in among the cells of the outer layer (Fig. 
16) then comes to lie close against the outer layer (Figs. 17 and 18), 
and finally jumps across a thin layer of mesenchyme into the medulla 
(Fig. 19). In some instances, the optic nerve after entering the medulla, 
can be followed as a fairly compact bundle of fibers for a considerable 
distance towards the anterior end of the brain, but in most of the 
specimens it soon disappears after entering the brain. 
In a few of these somewhat irregular transplanted eyes the optic 
nerve takes a very curious course passing across the cup cavity from 
the ganghonic layer through the pupil and then into the mesenchyme 
(see Figs. 20 and 21), ending there. In both of these experiments a 
small bundle of optic nerve fibers pierces the retina as far as the pig- 
ment layer. In transplanting these eyes the ganglionic layer was 
probably injured in such a way as to interfere with the normal path 
of the nerve fibers, and so they have probably followed the path of 
least resistance through the pupil and out into the mesenchyme. 
It would seem to me impossible to explain these various conditions 
of the optic nerve on any other basis than that they are outgrowths of 
nerve cells of the ganglonic layer of the retina. 
These experiments furnish important evidence in favor of the out- 
growth theory of the nerve fiber, and also that the fiber is a process 
of a nerve cell and not from a chain of cells. 
Nerves do not necessarily need predetermined paths, but may grow 
into the mesenchyme in various directions, probably along the path of 
least resistance. That under normal conditions normal nerves do follow 
predetermined paths is in no way disproyed by the experiments. 
The hap-hazard manner in which these various nerves grow into the 
mesenchyme without reaching any end organ or even apparently going 
towards any especial end organ, would indicate nerve fibers can grow out 
in the embryonic mesenchyme without being attracted there by a special 
chemotactic agent. 
